The related-art in-wheel motor drive device is disclosed in, for example, Patent Document 1 and Patent Document 2. The entire in-wheel motor drive device needs to be lightened and compactified to the extent possible because the entire in-wheel motor drive device needs to be accommodated inside a wheel and the weight and size of the in-wheel motor drive device affect the unsprung weight (driving performance) and the size of a passenger compartment space of a vehicle. Therefore, in the in-wheel motor drive devices of Patent Document 1 and Patent Document 2, at a position between a motor part configured to generate driving force and a wheel bearing part to be connected to a wheel, a speed reduction part configured to reduce a speed of rotation of the motor part to transmit the rotation to the wheel bearing part is provided so as to downsize the motor part and therefore downsize the entire in-wheel motor drive device. The above-mentioned motor part, wheel bearing part, and speed reduction part are held by a casing, and the casing is mounted to a vehicle body through intermediation of a suspension device (suspension) (not shown).
In the above-mentioned in-wheel motor drive device, a low-torque high-rotation (for example, about 15,000 min−1) motor is employed in the motor part so as to obtain a large torque required in the wheel bearing part while promoting light-weighting and compactification. Further, a cycloid reducer capable of obtaining a high speed reduction ratio with a compact size is employed in the speed reduction part.
In the motor part, there is employed a radial gap motor including a stator fixed to the casing, a rotor arranged inside the stator so as to be opposed thereto via a radial gap, and a rotation shaft of the motor, which has the rotor mounted to an outer periphery thereof to rotate integrally with the rotor. The rotation shaft of the motor has hollow structure, and is rotatably supported on the casing by rolling bearings.
The speed reduction part in which the cycloid reducer is utilized mainly includes an input shaft of the speed reducer having a pair of eccentric parts, a pair of curved plates arranged at the eccentric parts, respectively, a plurality of outer periphery engagement members configured to engage with outer peripheral surfaces of the curved plates to cause rotational motion of the curved plates, and a plurality of inner pins configured to transmit the rotational motion of the curved plates to an output shaft of the speed reducer. The above-mentioned rotation shaft of the motor is spline-connected to the input shaft of the speed reducer so that the torque is transmissible therebetween.